Abstract Transposons are mobile pieces of DNA that comprise significant proportions of eukaryotic genomes, including around 45% of the human genome and 30% of the fly genome. While most transposons have lost the ability to jump into new locations in the genome, several in each organism maintain the ability to mobilize. New transposon insertions are particularly consequential if they are generated in germline cells. Germline insertions are subsequently present in every cell of the organism they develop into and can be passed on to future generations. Thus, germline cells employ multilayered mechanisms to repress transposons. These repressive mechanisms are essential to largely suppress transposon for development of germline cells into mature eggs and sperm because transposon activity can contribute to sterility. Since these repressive mechanisms like the piRNA pathway render transposition events so rare, they are difficult to detect and our understanding of how transposons mobilize in the germline remains incomplete. Namely, the host factors that transposons employ to generate new insertions and the time point when they make new insertions during sperm maturation are undefined. To find potentially active transposons in the male germline, I have sequenced circular DNA from Drosophila testes with the piRNA pathway depleted. Disrupting the piRNA pathway allowed for transposon activation and revealed that the nomad transposon generates the most circular DNA in the testes. Circular DNA is a transposition intermediate of LTR retrotransposons and its presence can indicate the likelihood of a transposon being able to still make new insertions. With the knowledge that nomad is the most active transposon in the male germline, I propose to use this transposon to study how transposons mobilize in the germline. I hypothesize that transposons utilize host factors from the alt-EJ DNA repair pathway to achieve stage specific mobilization during spermatogenesis. I will utilize circular nomad DNA as a readout to find host factors required for its production and transposition reporters for nomad to precisely identify when it generates new insertions.